Unable to sign in in Xcode 27

[andriichernenko]

Xcode 27 is able to import this plugin, but signing in fails:

redirect_uri did not match any configured URIs. Passed URI: http://127.0.0.1:3118/callback

[zimeg]

👋 Hey @andriichernenko! Sorry to hear sign in is erroring 🔏 😓

I plan to explore this later but was wondering if these were the instructions for setup being followed? We might enhance the README with what works if so!

🔗 https://developer.apple.com/documentation/xcode/giving-external-agents-access-to-xcode#Configure-external-agents-to-use-the-MCP-server

[andriichernenko]

Hi @zimeg!

Integrating with MCPs via plugins is a new feature in Xcode 27 (the first beta of which was released yesterday during WWDC). There are 2 paragraphs of documentation on this so far: https://developer.apple.com/documentation/xcode/extending-and-customizing-agents#Install-agentic-coding-plug-ins

Xcode has an option to import plugins from Claude Code, and it worked with almost all plugins (including Figma and Atlassian), but not with Slack, which brought me to this repo. Xcode seems to pick up the .mcp.json config from the repo (at least changing the port in the config changed it in Xcode too).

I asked Claude about this, and the conclusion was that Xcode expects 127.0.0.1 as the callback URL host, but Slack expects localhost, so sign-in fails. I'm not sure if this makes sense, I don't know much about the process.

[gomera]

+1 here, just to give more info, I believe Apple is this case is doing what the RFC 8252 (OAuth 2.0 for Native Apps) recommends:
https://datatracker.ietf.org/doc/html/rfc8252#section-8.3

[gomera]

btw @andriichernenko I've just pushed a tiny script with a workaround that let you login and you can use the plugin. Obviously this is a temporary thing, I hope Slack provides a real fix more sooner than later.

https://github.com/gomera/slack-mcp-plugin-proxy